gpu: nova-core: register: add fields dispatcher internal rule

gpu: nova-core: register: add fields dispatcher internal rule

Fields are complex and cumbersome to match in a rule, and were only
captured in order to generate the field accessors. However, there are
other places (like the `Debug` and `Default` implementations) where we
would benefit from having access to at least some of the field
information, but refrained from doing so because it would have meant
matching the whole fields in a rule more complex than we need.

Introduce a new `@fields_dispatcher` internal rule that captures all the
field information and passes it to `@field_accessors`. It does not
provide any functional change in itself, but allows us to reuse the
captured field information partially to provide better `Debug` and
`Default` implementations in following patches.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250718-nova-regs-v2-10-7b6a762aa1cd@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>